The immune system must be able to discriminate between self and non-self. When self/non-self discrimination fails, the immune system destroys cells and tissues of the body and as a result causes autoimmune diseases. Regulatory T cells actively suppress activation of the immune system and prevent pathological self-reactivity and consequent autoimmune disease. Developing drugs and methods to selectively activate regulatory T cells for the treatment of autoimmune disease is the subject of intense research and, until the development of the present invention, has been largely unsuccessful.
Regulatory T cells (Treg) are a class of CD4+CD25+ T cells that suppress the activity of other immune cells. Treg are central to immune system homeostasis, and play a major role in maintaining tolerance to self-antigens and in modulating the immune response to foreign antigens. Multiple autoimmune and inflammatory diseases, including Type 1 Diabetes (T1D), Systemic Lupus Erythematosus (SLE), and Graft-versus-Host Disease (GVHD) have been shown to have a deficiency of Treg cell numbers or Treg function. Consequently, there is great interest in the development of therapies that boost the numbers and/or function of Treg cells.
One treatment approach for autoimmune diseases being investigated is the transplantation of autologous, ex vivo-expanded Treg cells (Tang, Q., et al, 2013, Cold Spring Harb. Perspect. Med., 3:1-15). While this approach has shown promise in treating animal models of disease and in several early stage human clinical trials, it requires personalized treatment with the patient's own T cells, is invasive, and is technically complex. Another approach is treatment with low dose Interleukin-2 (IL-2). Treg cells characteristically express high constitutive levels of the high affinity IL-2 receptor, IL2αβγ, which is composed of the subunits IL2RA (CD25), IL2RB (CD122), and IL2RG (CD132), and Treg cell growth has been shown to be dependent on IL-2 (Malek, T. R., et al., 2010, Immunity, 33:153-65). Clinical trials of low-dose IL-2 treatment of chronic GVHD (Koreth, J., et al., 2011, N Engl J Med., 365:2055-66) and HCV-associated autoimmune vasculitis patients (Saadoum, D., et al., 2011, N Engl J Med., 365:2067-77) have demonstrated increased Treg levels and signs of clinical efficacy. New clinical trials investigating the efficacy of IL-2 in multiple other autoimmune and inflammatory diseases have been initiated.
Proleukin (marketed by Prometheus Laboratories, San Diego, Calif.), the recombinant form of IL-2 used in these trials, is associated with high toxicity. Proleukin is approved for the treatment of Metastatic Melanoma and Metastatic Renal Cancer, but its side effects are so severe that its use is only recommended in a hospital setting with access to intensive care (http://www.proleukin.com/assets/pdf/proleukin.pdf). Until the more recent characterization of Treg cells, IL-2 was considered to be immune system stimulator, activating T cells and other immune cells to eliminate cancer cells. The clinical trials of IL-2 in autoimmune diseases have employed lower doses of IL-2 in order to target Treg cells, because Treg cells respond to lower concentrations of IL-2 than many other immune cell types because of their expression of IL2Rαβγ (Klatzmann D, 2015 Nat Rev Immunol. 15:283-94). However, even these lower doses resulted in safety and tolerability issues, and the treatments used have employed daily subcutaneous injections, either chronically or in intermittent 5 day treatment courses. Therefore, there is need for an autoimmune disease therapy that potentiates Treg cell numbers and function, that targets Treg cells more specifically than IL-2, that is safer and more tolerable, and that is administered less frequently.
One approach to improving the therapeutic index of IL-2-based therapy is to use variants of IL-2 that are selective for Treg cells relative to other immune cells. IL-2 receptors are expressed on a variety of different immune cell types, including T cells, NK cells, eosinophils, and monocytes, and this broad expression pattern likely contributes to its pleiotropic effect on the immune system and high systemic toxicity. The IL-2 receptor exists in three forms: (1) the low affinity receptor, IL2RA, which does not signal; (2) the intermediate affinity receptor (IL2Rβγ), composed of IL2RB and IL2RG, which is broadly expressed on conventional T cells (Tcons), NK cells, eosinophils, and monocytes; and (3) the high affinity receptor (IL2αβγ), composed of IL2RA, IL2RB, and IL2RG, which is expressed transiently on activated T cells and constitutively on Treg cells. IL-2 variants have been developed that are selective for IL2Rαβγ relative to IL2Rβγ (Shanafelt, A. B., et al., 2000, Nat Biotechnol. 18:1197-202; Cassell, D. J., et. al., 2002, Curr Pharm Des., 8:2171-83). These variants have amino acid substitutions which reduce their affinity for IL2RB. Because IL-2 has undetectable affinity for IL2RG, these variants consequently have reduced affinity for the IL2Rβγ receptor complex and reduced ability to activate IL2Rβγ-expressing cells, but retain the ability to bind IL2RA and the ability to bind and activate the IL2Rαβγ receptor complex. One of these variants, IL2/N88R (Bay 50-4798), was clinically tested as a low-toxicity version of IL-2 as an immune system stimulator, based on the hypothesis that IL2Rβγ-expressing NK cells are a major contributor to toxicity. Bay 50-4798 was shown to selectively stimulate the proliferation of activated T cells relative to NK cells, and was evaluated in phase I/II clinical trials in cancer patients (Margolin, K., et. al., 2007, Clin Cancer Res., 13:3312-9) and HIV patients (Davey, R. T., et. al., 2008, J Interferon Cytokine Res., 28:89-100). These trials showed that Bay 50-4798 was considerably safer and more tolerable than Proleukin, and also showed that it increased the levels of CD4+ T cells and CD4+CD25+ T cells in patients. However, the increase in CD4+ T cells and CD4+CD25+ T cells were not indicative of an increase in Treg cells, because identification of Tregs requires additional markers in addition to CD4 and CD25, and because Treg cells are a minor fraction of CD4+CD25+ cells. Subsequent to these trials, research in the field more fully established the identity of Treg cells and demonstrated that Treg cells selectively express IL2αβγ (reviewed in Malek, T. R., et al., 2010, Immunity, 33:153-65). Based on this new research, it can now be understood that IL2Rαβγ selective agonists should be selective for Treg cells.
A second approach to improving the therapeutic index of an IL-2 based therapy is to optimize the pharmacokinetics of the molecule to maximally stimulate Treg cells. Early studies of IL-2 action demonstrated that IL-2 stimulation of human T cell proliferation in vitro required a minimum of 5-6 hours exposure to effective concentrations of IL-2 (Cantrell, D. A., et. al., 1984, Science, 224: 1312-1316). When administered to human patients, IL-2 has a very short plasma half-life of 85 minutes for intravenous administration and 3.3 hours subcutaneous administration (Kirchner, G. I., et al., 1998, Br J Clin Pharmacol. 46:5-10). Because of its short half-life, maintaining circulating IL-2 at or above the level necessary to stimulate T cell proliferation for the necessary duration necessitates high doses that result in peak IL-2 levels significantly above the EC50 for Treg cells or will require frequent administration (FIG. 1). These high IL-2 peak levels can activate IL2Rβγ receptors and have other unintended or adverse effects. An IL-2 analog with a longer circulating half-life than IL-2 can achieve a target drug concentration for a specified period of time at a lower dose than IL-2, and with lower peak levels. Such an IL-2 analog will therefore require either lower doses or less frequent administration than IL-2 to effectively stimulate Treg cells. Indeed, in cynomolgus monkeys dosed with an IgG-IL2 fusion protein with a circulating half-life of 14 hours stimulated a much more robust increase in Tregs compared to an equimolar dose of IL-2 (Bell, et al., 2015, J Autoimmun 56:66-80). Less frequent subcutaneous administration of an IL-2 drug will also be more tolerable for patients. A therapeutic with these characteristics will translate clinically into improved pharmacological efficacy, reduced toxicity, and improved patient compliance with therapy.
One approach to extending the half-life of therapeutic proteins is to fuse the therapeutically active portion of the molecule to another protein, such as the Fc region of IgG, to increase the circulating half-life. Fusion of therapeutic proteins with IgG Fc accomplishes this by increasing the hydrodynamic radius of the protein, thus reducing renal clearance, and through Neonatal Fc Receptor (FcRn)-mediated recycling of the fusion protein, thus prolonging the circulating half-life. The fusion of therapeutic proteins to albumin (Sleep, D., et. al., 2013, Biochem Biophys Acta., 1830:5526-34) and nonimmunogenic amino acid polymer proteins (Schlapschy, M., et. al., 2007, Protein Eng Des Sel. 20:273-84; Schellenberger, V., et. al., 2009, Nat Biotechnol. 27:1186-90) have also been employed to increase circulating half-life. However, construction of such fusion proteins in a manner that ensures robust biological activity of the IL2 Selective Agonist fusion partner can be unpredictable, especially in the case of an IL-2 Selective Agonist, which is a small protein that is defective in binding to one of the receptor subunits and that must assemble a complex of three receptor subunits in order to activate the receptor (Wang, X., et al., 2005, Science 310:1159-63).
Other researchers have created various IL-2 fusion proteins, using wild-type IL-2 or IL-2 with a C125S substitution to promote stability. Morrison and colleagues (Penichet, M. L., et., al., 1997, Hum Antibodies. 8:106-18) created a fusion protein with IgG fused to wild-type IL-2 to both increase the circulating half-life of IL-2 and to target IL-2 to specific antigens for the purpose of potentiating the immune response to the antigen. This fusion protein consisted of an intact antibody molecule, composed of heavy (H) and light (L) chains, wherein the N-terminal H chain moiety was fused to a C-terminal IL-2 protein moiety. This IgG-IL-2 fusion protein possessed Fc effector functions. Key effector functions of IgG Fc proteins are Complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC). The IgG-IL-2 fusion protein was highly active in an IL-2 bioassay and was shown to possess CDC activity. Thus, Penichet et. al. taught the use of antibody-IL2 fusion proteins to target IL-2 activity to antigens recognized by the antibody, for the purpose of potentiating humoral and cell-mediated immune responses to the antigen. In a similar manner, Gillies and colleagues have constructed a number of IgG-IL-2 fusion proteins for cancer immunotherapy, utilizing the antibody portion of the fusion protein to target tumor antigens, and the IL-2 portion to stimulate the immune response to tumor cells (reviewed in Sondel, P. M., et. al., 2012, Antibodies, 1:149-71). These teachings are quite distinct from the present inventive technology, wherein an IL-2 selective agonist, which promotes the growth and activity of immunosuppressive Treg cells, is fused with an effector function-deficient Fc protein moiety for the purpose increasing systemic exposure.
Strom and his colleagues have constructed fusion proteins with IL-2 fused to the N terminus of an Fc protein for the purpose of eliminating activating T cells expressing the high-affinity IL-2 receptor (Zheng, X. X., et al., 1999, J Immunol. 1999, 163:4041-8). This fusion protein was shown to inhibit the development of autoimmune diabetes in a T cell transfer mouse model of T1D. The IL2-Fc fusion protein was shown to inhibit the function of disease-promoting T cells from T1D-susceptible female NOD mice when transplanted into less disease-susceptible male NOD mice. They also demonstrated that the IL-2-Fc fusion protein could kill cells expressing the high-affinity IL-2 receptor in vitro. These investigators further compared IL2-Fc fusion proteins constructed from an Fc derived from an effector function-competent IgG2b Fc and a mutated effector function-deficient IgG2b Fc. Only the IL2-Fc fusion protein containing the effector function-competent Fc was efficacious in preventing disease onset. Thus, these investigators teach that an IL2-Fc fusion protein with effector functions can eliminate disease-causing activated T cells, and that Fc effector functions are necessary for its therapeutic activity. These teachings are quite distinct from the present inventive technology, wherein an IL-2 selective agonist, which promotes the growth and activity of immunosuppressive Treg cells, is fused with an effector function-deficient Fc protein moiety for the purpose increasing systemic exposure and optimizing Treg expansion. Other work from Strom and colleagues teaches the use of a IL2-Fc fusion protein in promoting transplant tolerance (Zheng, X. X., et al., 2003, Immunity, 19:503-14). In this work, an IL2-Fc fusion protein is used in a “triple therapy” in which it is combined with an IL15-Fc receptor antagonist and rapamycin. Again, these investigators teach that the IL2-Fc fusion protein must have Fc effector functions to be efficacious, and further teach that this IL-2-Fc fusion protein must be combined with two other molecules in order to be efficacious.
This invention provides for a novel therapeutic agent, an IL2 Selective Agonist-Fc fusion protein with a peptide linker of from 6-30 amino acids. This configuration combines the high cellular selectivity of a IL2 Selective Agonist for Treg cells with a long circulating half-life. In the course of developing this molecule, there were surprising and unexpected findings that revealed structural elements and design features of the protein that are essential for bioactivity, and that led to the discovery of several novel proteins that fulfill the desired therapeutic characteristics.